"Sorrow knocked at my door, but I was afraid" is an example of <em>D. personification.</em>
Personification is a literary device in which animals, objects or ideas are given human attributes. In this case, sorrow is given the ability to knock at someone's door, but as we know it is only a figure of speech because sorrow cannot knock at the door.
Bob Ewell was killed by Boo Radley, who stuck a kitchen knife into his chest on the night of Halloween. Bob wanted to get revenge on Atticus for trying to kill the children who were walking back from a Halloween school party.